Electronic iris controllers have been developed to eliminate the need for or supplement a mechanical iris to control the amount of light that impinges on a solid state electronic image sensor. An electronic iris controls the exposure time by periodically exposing or removing the charge generated by the photon-generated hole-electron pairs within the image sensor. This process of periodic destruction or removal of charge is referred to as the clearing of the image sensor. The charge is also removed from the image sensor when the signal from the image sensor is transferred to an image memory associated with the image sensor. This process is referred to as the image transfer. The length of time between the clearing of the image and the transfer of the image sets the electronic exposure time. By altering the relative positions of the image clear operation and the image transfer operation, the length of exposure time can be controlled electronically.
The image memory is capacitively coupled to the image sensor. Accordingly, during the readout of information from the image memory, no other operations are performed on the image sensor or the image memory to minimize interference with the readout sequence. For most television applications, the maximum exposure time for a field of data is 16.67 milliseconds. A typical field of television data comprises 262.5 lines of information. This sets the minimum exposure at 63.5 microseconds for operations during the readout of the image memory, due to the fact that all image clear or image transfer operations must occur during the horizontal blanking period or vertical blanking period. Iris control has typically been limited to exposure adjustments in 63.5 microseconds increments only. This incremental change of exposure time creates a problem in that the percentage change of exposure is extremely non-linear in that the initial incremental changes are very small compared to the entire 16.67 millisecond field time and the final incremental changes can result in exposure changes up to and including fifty percent.